Go into cell formating, patterns.  Make sure "patterns" is
on "automatic" and make sure fill color is the upper left
selection (black.)

If that doesn't work, then your printer may not be able to
print pure black.  Printers that don't have a black ink
cartridge have to combine the colors to print dark,
possibly one color is printing more than the others.

Also, you could try deleting the printer, rebooting, then
reinstalling the printer.  Make sure your print driver is
up to date before doing this, though.

Good Luck!